Phosphorus has many physiologic functions in the body. Roughly 85% of P is in bone and teeth, 14% in soft tissue, and 1% in extracellular fluid. Fat corrected milk (4%) contains 1 g of P/ kg. Blood plasma normally ranges from 4 to 8 mg/dl of inorganic P, 6 to 8 times lower than whole blood. There is no regulation of blood P, thus P input from the intestine or bone must be continuous to maintain P concentrations in plasma. Endogenous P loss occurs through feces and is approximately 13 g/d in an adult dairy cow. Bone P can support deficits in early lactation when total cumulative deficit may be 600 to 1,000 g of P. Phosphorus is absorbed from the small intestine by active and passive processes and is sensitive to 1,25-dihydoxycholecalciferol. Aphosphorosis was recognized as a clinical syndrome in cattle at the turn of the century. Clinical signs included ill-thrift, lameness, softening or fracturing of bones, depraved appetite, and poor reproduction. Calves nursing cows grew poorly. Milk cows were more often affected than nonlactating cows. The etiology was low forage P content (<.15%). Anecdotal reports associated low P intake in lactating dairy cows with poor reproduction, poor estrus expression, cystic ovaries and low conception rate. In a summary of sixteen controlled studies ranging in dietary phosphorus content from .16% to .56%., there was no effect of phosphorus content on CR, percent of estruses detected, or days to first insemination. Dairy cattle tolerate dietary phosphorus content between .20 to .30%, however milk production has typically been lower compared to cows receiving greater than .30% dietary phosphorus. It seems dairy cattle can safely be fed dietary phosphorus contents of .33 to .40% with no negative effects on reproduction or milk production.